1. Field of the Invention
This invention relates to removal of contaminants from soil, and more specifically, the use of radiofrequency (RF) ground heating to remove contaminants from soil in-situ.
2. Description of Related Art
Bridges and Taflove of the Illinois Institute of Technology Research Institute (IITRI) proposed mining a shaft through material above oil shale, known as overburden, to the top of the oil shale and inserting an array of electrodes into the oil shale starting from this shaft. This method for RF heating of oil shale is described in U.S. Pat. No. 4,449,585, "Apparatus and Method For In-situ Controlled Heat Processing of Hydrocarbonaceous Formations" by J. Bridges and A. Taflove issued Mar. 22, 1984. Their electrode array is designed to be a "triplate," where the center electrode row is at high potential and the adjacent rows on either side at ground potential.
A somewhat different method of RF shale heating utilizes an array of specially designed dipole antennas inserted into the ground, described in U.S. Pat. No. 4,140,179, "In-situ Radio Frequency Selective Heating Process" by R. S. Kasevich, M. Kolker and A. S. Dwyer issued Feb. 20, 1979. A problem with this approach is that the antenna elements must be matched to the electrical conditions of the surrounding formation. As the formation is heated, the electrical conditions can change, and the dipole antenna elements have to be removed and changed, which presents significant practical and economic difficulties.
The triplate RF electrode arrangement was applied to contaminated soil as described in "Test of Radio Frequency In Situ Heating for Treatment of Soil at Rocky Mountain Arsenal" by R. Snow, G. Sresty, C. Conroy R. Collins, and T. Kilgannon, H. Der, J. Enk, J. Bridges, Proceedings of the XIV Superfund Conference, Nov. 30-Dec. 2, 1993, Washington, DC ("Superfund report"). This report describes the removal of organic contaminants from soil in situ by heating soil with RF antennas and power amplifiers. The PCBs are volatilized and then collected at the surface of the soil for disposal by a number of methods, for example catalytic combustion or adsorption on activated charcoal.
It is asserted in the Superfund report, id. that the "triplate" has the center row at a high electrical potential and the outer electrode rows at "ground" potential. However, since the triplate array is at the end of some length of RF cable and other wires, all the electrodes (center row and outside rows) will have sinusoidally varying voltages consistent with maintaining some average voltage. Since all electrodes will have a fluctuating voltage, it is likely that these voltages will cause currents to flow to other ground points, for example at the control station, which would produce voltages that can pose a hazard to operating personnel. The triplate arrangement depends on symmetry to prevent such Currents, and such symmetry is easily broken.
Another problem with the triplate is its method of feeding, which is via wires originating in a single feed point as shown in U.S. Pat. No. 4,449,585 mentioned above. Since the feed wires have different lengths and therefore different, significant inductive reactances, the voltages at various electrodes will vary and will produce uneven currents and consequent uneven heating.
Currently there is a need for a safe and efficient method of using RF energy to remove contaminants; from soil.